1. Field of the Invention
The present invention relates to methods for the mechanochemical preparation of stable passivated nanoparticles, made of e.g. silicon or germanium.
2. General Background of the Invention
A nanoparticle (or nanopowder) is a microscopic particle with at least one dimension less than 100 nanometer (nm). Nanoparticles have recently been at the forefront of biomedical, optical, and electronics research because they can exhibit fundamentally new behavior when their sizes fall below the critical length scale associated with any given property. A bulk material is generally considered to have uniform physical properties throughout regardless of its size, but at the nano-scale the properties of materials change as the percentage of atoms at the surface of the material becomes significant. Below the micrometer scale, size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles, and supermagnetism in magnetic materials.
Quantum confinement occurs when electrons and holes in a semiconductor are restricted in one or more dimensions. A quantum dot is confined in all three dimensions, a quantum wire is confined in two dimensions, and a quantum well is confined in one dimension. That is, quantum confinement occurs when one or more of the dimensions of a nanocrystal is made very small so that it approaches the size of an exciton in bulk crystal, called the Bohr exciton radius. An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor. An exciton is an elementary excitation, or a quasiparticle of a solid. A quantum dot is a structure where all dimensions are near the Bohr exciton radius, typically a small sphere. A quantum wire is a structure where the height and breadth is made small while the length can be long. A quantum well is a structure where the height is approximately the Bohr exciton radius while the length and breadth can be large. Quantum confinement effects at very small crystalline sizes can cause silicon and germanium nanoparticles to fluoresce, and such fluorescent silicon and germanium nanoparticles have great potential for use in optical and electronic systems as well as biological applications.
Silicon and germanium nanoparticles may be used, e.g., in optical switching devices, photovoltaic cells, light emitting diodes, lasers, and optical frequency doublers, and as biological markers.
The photoluminescence (PL) mechanism in silicon and germanium nanoparticles is also influenced by the nature and bonding state of the particle surface. Photoluminescence is a process in which a chemical compound absorbs photons (electromagnetic radiation), thus transitioning to a higher electronic energy state, and then radiates photons back out, returning to a lower energy state. The period between absorption and emission is typically extremely short, on the order of 10 nanoseconds. Under special circumstances, however, this period can be extended into minutes or hours. Ultimately, available chemical energy states and allowed transitions between states (and therefore wavelengths of light preferentially absorbed and emitted) are determined by the rules of quantum mechanics. A basic understanding of the principles involved can be gained by studying the electron configurations and molecular orbitals of simple atoms and molecules. More complicated molecules and advanced subtleties are treated in the field of computational chemistry.
Light absorption and emission in a semiconductor are known to be heavily dependent on the detailed band structure of the semiconductor. Direct band gap semiconductors are semiconductors for which the minimum of the conduction band occurs at the same wave vector, k, as the maximum of the valence band. Direct band gap semiconductors have a stronger absorption of light as characterized by a larger absorption coefficient and are also the favored semiconductors when fabricating light emitting devices. Indirect band gap semiconductors are semiconductors for which the minimum of the conduction band does not occur at the same wave vector as the maximum of the valence band. Indirect band gap semiconductors are known to have a smaller absorption coefficient and are rarely used in light emitting devices.
This striking difference between direct band gap semiconductors and indirect band gap semiconductors can be explained by the energy and momentum conservation required in the electron-photon interaction. The direct band gap semiconductor has a vertically aligned conduction and valence band. Absorption of a photon is obtained if an empty state in the conduction band is available for which the energy and momentum equals that of an electron in the valence band plus that of the incident photon. Photons have little momentum relative to their energy since they travel at the speed of light. The electron therefore makes an almost vertical transition on the E-k diagram. For an indirect band gap semiconductor, the conduction band is not vertically aligned to the valence. Therefore a simple interaction of an incident photon with an electron in the valence band will not provide the correct energy and momentum corresponding to that of an empty state in the conduction band. As a result absorption of light requires the help of another particle, namely a phonon.
A phonon is a particle associated with lattice vibrations and has a relatively low velocity close to the speed of sound in the material. Phonons have a small energy and large momentum compared to that of photons. Conservation of both energy and momentum can therefore be obtained in the absorption process if a phonon is created or an existing phonon participates. The minimum photon energy that can be absorbed is slightly below the band gap energy in the case of phonon absorption and has to be slightly above the band gap energy in the case of phonon emission. Since the absorption process in an indirect band gap semiconductor involves a phonon in addition to the electron and photon, the probability of having an interaction take place involving all three particles will be lower than a simple electron-photon interaction in a direct band gap semiconductor. As a result one finds that absorption is much stronger in a direct band gap material. Similarly, in the case of light emission, a direct band gap material is also more likely to emit a photon than an indirect band gap material. While indirect band gap materials are occasionally used for some LEDs, they result in a low conversion efficiency. Direct band gap materials are used exclusively for semiconductor laser diodes.
The presence of oxygen at a silicon surface has been shown to have deleterious effects on luminescence properties. In a study conducted in 1999 at the University of Rochester, scientists hypothesized that oxygen at the surface of a porous silicon (PSi) nanoparticle diminished photoluminescence. PSi samples with varying porosities were kept at room temperature in either Argon (Ar) atmosphere or air. Investigating the evolution of the chemical coverage of an Ar-stored sample as it was exposed to air, researchers discovered through Fourier Transform Infrared Spectrometry (FTIR) analysis that hydrogen-passivated PSi samples that initially showed no sign of oxygen absorption showed Si—O—Si peaks in as little as 3 minutes after exposure to air. After 24 hours, the Si—H peaks disappeared and the Si—O—Si and Si—O—H peaks dominated spectra. When the samples were exposed to air for longer than 200 minutes, no significant change in the Si—O—Si and Si—O—H peaks was observed, indicating stabilization of the surface chemical coverage. As the surface passivation was gradually changing, the PL was redshifted. It was concluded that both porosity (or size) and chemical coverage dictate the recombination mechanism. The results suggest that the electron-hole recombination in samples exposed to oxygen occurs via carriers trapped in oxygen-related localized states that are stabilized by the widening of the gap induced by quantum confinement.
Surface modification of nanoparticles with alkyl groups has been demonstrated by chemical reactions on Si—H and Si-Halide capped surfaces but with limited success. Although PL is initially preserved, incomplete alkylation by these two-step techniques ultimately leads to non-uniform coverage and instability with respect to oxidation. Given the high affinity of silicon for oxygen, it is therefore necessary to utilize a particle surface passivation technique that can be conducted in an oxygen-free environment and that facilitates direct interaction of the alkyl groups with surface silicon atoms.
Current methods of Si—C bond formation on silicon surfaces involve either the use of a well-defined clean silicon surface maintained under ultrahigh vacuum conditions, the use of chemical or electrochemical etching of the silicon surface, or the Wurtz reaction of halosilanes. Wet chemistry approaches, such as those requiring use of hydrogen fluoride etches or condensation of halosilanes, involve unstable hydrogen- or halogen-terminated surface intermediates and the use of corrosive or toxic chemicals. Similarly, current direct reaction methods involve the use of expensive equipment and may be difficult to scale. These direct approaches, commonly involving the mechanical scribing of silicon in the presence of reactive organic reagents, have found success in the patterning of silicon surfaces through reaction of a freshly exposed surface with the organic reagent. These techniques are limited to large and regular surfaces and are not practical for use with nanoparticles.
Niederhauser et al. of Brigham Young University in Provo, Utah developed a method for preparing alkyl monolayers on silicon, which consists of cleaning a silicon wafer to remove adventitious contaminants from its surface, leaving its thin native oxide layer, wetting the dry surface of the clean silicon with an unsaturated, organic molecule, mechanically scribing the silicon with a diamond-tipped instrument while it is wet with the unsaturated, organic liquid, and cleaning the scribed surface to remove excess organic liquid and silicon particles that are produced by scribing. Their process is the first known to use wet-chemical preparation of monolayers on silicon that does not require a hydrogen-terminated silicon intermediate.
Current methods of silicon surface functionalization, including Niederhauser's, have numerous shortcomings. Niederhauser's approach is applicable only to flat surfaces. Even those processes that apply to the formation of functionalized silicon nanoparticles require multistep processes involving the use of corrosive or toxic reagents or potentially explosive reaction conditions. The initially formed silicon nanoparticles typically result from the reduction of silicon halides, the thermal or laser decomposition of silanes, the oxidation of metal silicides, or the electrochemical etching of bulk silicon. Each procedure uses either a corrosive or very reactive reagent and the initially formed nanoparticles are highly reactive due to hydrogen or halide terminated surfaces.
There is a need for simple, direct methods of producing stable passivated silicon nanoparticles. The present invention meets this need by providing a simple one-step process for the formation and passivation of a silicon surface under very mild conditions. In addition, the process does not need to involve additional solvent and can be conducted on a continuous basis. This process constitutes a novel method of producing stable passivated silicon nanoparticles that do not exhibit the shortcomings of any of the existing methods, yet provide the novel aspects of quantum confinement effects.
The following references, and all references mentioned herein, are incorporated herein by reference:
U.S. Pat. No. 7,371,666 entitled “Process for producing luminescent silicon nanoparticles”;
U.S. Pat. No. 6,132,801 entitled “Producing Coated Particles by Grinding in the Presence of Reactive Species” and which uses a mortar and pestle in a controlled dry box (see column 5, lines 37-61);
U.S. Pat. No. 5,702,060 entitled “High-Energy High-Capacity Oscillating Ball Mill”;
U.S. Pat. No. 6,444,009 entitled “Method for producing environmentally stable reactive alloy powders”; and
Castro et al., “Nanoparticles from Mechanical Attrition”, Chapter 1 of Synthesis, Functionalization and Surface Treatment of Nanoparticles, (American Scientific Publishers 2002).
The present inventors are aware of references which discuss ball milling of silicon to produce nanoparticles; however, to the knowledge of the present inventors none of these references discuss simultaneous formation of nanoparticles and passivation of the nanoparticles with a reactive medium.